Remove another unsafe cast.

m_fStretchTexCoordVelocityX -> TexCoordVelocityX, etc.
Remove TYPE_STRETCH.
Disassociate BGAL::Type from the "Type" BGA metric.  Prefer "Type=sprite",
"Type=particles", "Type=tile", and "Stretch=1" (for sprite only).  Type=0, 1, 2, 3 still
work; 0 is "Type=sprite", Stretch=0 and 1 is Type=sprite, Stretch=1.
Remove old code.
Historical note: "stretch" was used in this code at various times to mean both 1:
"stretch the internal texture image to fit the texture" and 2: "stretch the displayed
image to fill the screen".  It now only means #2.  #1 is done automatically when needed.
Note that LoadFromAniLayerFile should never load an INI; it's for backwards-
compatibility only.  LoadFromIni is simpler and more flexible.

stepmania/src/BGAnimationLayer.h

@@ -78,11 +78,9 @@ protected:
 	enum Type
 	{
 		TYPE_SPRITE,
-		TYPE_STRETCH,
 		TYPE_PARTICLES,
 		TYPE_TILES,
 		NUM_TYPES,
-		TYPE_INVALID
 	} m_Type;
 
 
@@ -101,8 +99,8 @@ protected:
 	bool m_bLighting;
 
 	// stretch stuff
-	float m_fStretchTexCoordVelocityX;
-	float m_fStretchTexCoordVelocityY;
+	float m_fTexCoordVelocityX;
+	float m_fTexCoordVelocityY;
 
 	// particle and tile stuff
 	float m_fZoomMin, m_fZoomMax;